Windows: Fix ssize_t unconditionally defined

The standard procedure to define ssize_t on Windows is to wrap it around
a check for the _SSIZE_T_DEFINED macro.

If not done, it makes it impossible to use the libusb.h header along
with other headers (from other libraries) that also attempt to define
ssize_t.

Closes #1007

Signed-off-by: Paul Cercueil <paul@crapouillou.net>